#134 - The Big Flat sits on Flaxville Formation

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All that's left . . .
The Flaxville Formation is made of gravels and sands that were deposited by large braided streams flowing eastward from the Rocky Mountains millions of years ago (Miocene-Pliocene, 2.5-10 million years ago). The Flaxville Formation experienced significant erosion during the late Pliocene and Quaternary periods, a process that continues today. This erosion dissected a once-widespread plain of fluvial (river) gravels, leaving only the isolated higher-elevation plateaus seen today - shown as the pale green patches on the map above.

The Big Flat . . .
The small farming communities of Hogeland and Turner sit on the remnant called the Boundary Plateau, known to locals as "The Big Flat". Most of the gravel beneath the soils of the Big Flat is unconsolidated (not cemented together). However, along Cherry Ridge 7 miles west of the plateau, the Flaxville Formation outcrops as cliffs of conglomerate - an unusual type of sedimentary rock made of gravel held together by a natural cement (photos below). Cherry Ridge is 20 miles north of Zurich (Zurich is 36 miles east of Havre).

Source of the gravel? . . .
The gravel consists of well-rounded pebbles from less than an inch to a foot or more diameter, of quartzite and the familiar red and green argillite from Rocky Mountains (shown in photo below, click to enlarge). Limestone, also from the Rockies, may have been dissolved and redeposited as the cementing material holding the gravel together. The conglomerate that form the cliffs here on Cherry Ridge north of Zurich has been thoroughly cemented with calcite - the same mineral that limestone is made of.

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Just high enough . . .
The Laurentide Ice Sheet covered this area during the last two glacial periods, growing about as far south as the Missouri River. The second to last advance, known as the Illinoian Ice Age, peaked 190,000-130,000 years ago, and the last one, called the Wisconsin Ice Age, reached its maximun 25,000-20,000 years ago. Although the ice sheet grew over the Big Flat during the Illinoian Ice Age, the glacier flowed around it during the more recent Wisconsin Ice Age. As a result, soil on the Big Flat was given over 100,000 extra years to develop, compared to soils on the surrounding area. This, combined with the fact that the surface is flat, makes the boundary plateau a good place to grow crops. The image below shows the correlation - The Big Flat (outlined in yellow) is covered with farmland. In some places the Flaxville gravels beneath the Big Flat are saturated with an abundance of groundwater, making center-pivot irrigation possible - another plus for farmers.

Term: fluvial